1. Field of the Invention
The present invention generally relates to an aluminum/ceramic bonding substrate having a ceramic substrate and an aluminum plate bonded to the ceramic substrate, and a method for producing the same. More specifically, the invention relates to an aluminum/ceramic substrate used as an aluminum/ceramic insulating substrate for a power module having a high heat cycle resistance, a method for producing the same, and a power module using the aluminum/ceramic bonding substrate.
2. Description of the Prior Art
Conventionally, a copper plate is used as a metal circuit plate of a metal/ceramic insulating substrate for a power module. In recent years, in order to realize a higher heat cycle resistance, it has been proposed to use an aluminum plate as a metal circuit plate of a metal/ceramic insulating substrate, and such a substrate has been put to practical use. It is guessed that the reasons why a high heat cycle resistance can be obtained by the use of an aluminum plate are as follows. Since aluminum has a lower yield stress than that of copper, aluminum enhance the function of relaxing thermal stress, which is caused on the bonding interface between a metal and a ceramic, by the plastic deformation of the metal itself. Therefore, damage to a ceramic substrate due to a heat cycle applied to a metal/ceramic bonding substrate decreases to improve the heat cycle resistance.
However, it has been considered that an oxide film is formed on the surface of an aluminum plate to obstruct the bonding of the aluminum plate to a ceramic substrate, so that it is difficult to bond the aluminum plate directly to the ceramic substrate unless the oxide film on the surface of the aluminum plate is removed by a special treatment to maintain a clean surface.
Therefore, an aluminum plate is generally bonded to a ceramic substrate via an Al—Si brazing filler metal. As an example of such a method, there is known a method for arranging an aluminum plate on a ceramic substrate via an Al—Si brazing filler metal to heat them in vacuo to bond the aluminum plate to the ceramic substrate via the brazing filler metal (see, e.g., Japanese Patent Laid-Open No. 2001-168482)
On the other hand, as a method for bonding an Si containing aluminum plate directly to a ceramic substrate without the need of any intermediate materials, such as a brazing filler metal, there is known a so-called eutectic bonding method for heating an Si containing aluminum plate and a ceramic substrate in an inert atmosphere at a temperature between their eutectic point and the melting point of aluminum to produce an Al—Si eutectic melt between the Si containing aluminum plate and the ceramic plate to bond the Si containing aluminum plate directly to the ceramic substrate (see, e.g., Japanese Patent Laid-Open No. 52-3791).
There is also known a method for bonding an Al—Si alloy plate containing 50 wt % or less of Si directly to a silicon nitride (Si3N4) substrate (see, e.g., Japanese Patent Laid-Open No. 2001-168482).
However, in the method for bonding an aluminum plate to a ceramic substrate via a brazing filler metal, it is required to carry out a brazing filler metal paste applying step of previously applying a paste-like brazing filler metal on the ceramic substrate or aluminum plate if the paste-like brazing filler metal is used, and it is required to carrying out a brazing filler metal foil inserting step of inserting a brazing filler metal foil, which is not a principal component of a product, when the aluminum plate and the ceramic substrate are stacked if the brazing filler metal foil is used, so that there are problems in that the number of steps increases and costs, such as material costs, increase. In addition, it is required to bond them in a high vacuum in a vacuum furnace, and it is not possible to bond them in a continuous furnace, such as a belt type tunnel kiln, which has a high productivity, so that there is a problem in that production costs are relatively high. Moreover, if dissimilar elements are dispersed and mixed in aluminum from the brazing filler metal, the yield stress of aluminum generally increases to deteriorate the function of relaxing thermal stress of the aluminum/ceramic bonding substrate by the plastic deformation of aluminum, so that there is a problem in that the heat cycle resistance of the aluminum/ceramic bonding substrate deteriorates.
The eutectic bonding method can be only used for bonding an aluminum plate to a ceramic substrate which produces an eutectic melt with aluminum, so that it is difficult to bond an Si containing aluminum to a ceramic which does not produce an Al—Si eutectic melt with the Si containing aluminum.
In the method for bonding an Al—Si alloy plate to a silicon nitrogen substrate, if dissimilar elements, such as Si, are excessively mixed in aluminum, the yield stress of aluminum generally increase to deteriorate the function of relaxing thermal stress of the aluminum/ceramic bonding substrate by the plastic deformation of aluminum, so that there is a problem in that the heat cycle resistance of the aluminum/ceramic bonding substrate deteriorates. Therefore, the purity of aluminum is preferably high. In addition, the use of this method is limited since the silicon nitride (Si3N4) substrate is expensive and has a lower heat sinking ability than aluminum nitride. Therefore, it is desired to provide a method for bonding a pure aluminum plate containing no dissimilar elements directly to aluminum nitride, which has excellent heat sink characteristics, or aluminum oxide which is inexpensive and easily available although its heat sink ability is lower than that of aluminum nitride. However, such a method is not known, and a brazing filler metal is used for bonding a pure aluminum plate to an aluminum nitride substrate or an aluminum oxide substrate.